An optical waveguide circuit, especially, a Planar Lightwave Circuit (PLC) in which an optical waveguide was formed on a planar surface has been extensively used as a key device for supporting a recent optical communication network system. In particular, passive device such as an optical multi/demultiplexer or an optical branch device using a silica optical waveguide has become indispensable for practical application of a low-priced and high-performance system in the fields from a backbone network represented by a large-capacity optical communication to an access-based network and have already been put into practical use and commercial mass production.
As an example of the PLC, an Arrayed Waveguide Grating (AWG) is shown in FIGS. 27 and 28. FIG. 27 is a top view of the AWG device; and FIG. 28 is a cross-sectional view taken along the line VIIIb—VIIIb in FIG. 27. This device has, at the inside of cladding layers thereof constituted by lower cladding 802 and upper cladding 804 formed on substrate 801, a waveguide 811 that propagates a wavelength-multiplexed optical signal, a first slab waveguide 812 connected to the waveguide 811, a waveguide 815 that separately propagates optical signals of different wavelengths, a second slab waveguide 814 connected to the waveguide 815, and arrayed waveguides 813 that connect the first slab waveguide 812 and second slab waveguide 815. The device has a function of demultiplexing the wavelength-multiplexed optical signal into different wavelengths, or contrary, multiplexing optical signals of different wavelengths onto one optical fiber. In this configuration, as shown in FIG. 28, the upper portions of cores 803 and gaps between the cores 803 in the arrayed waveguides 813 are covered by the upper cladding 804.
An operational principle of the device will be briefly described below with the case of demultiplex taken as an example.
A wavelength-multiplexed optical signal incident into the waveguide 811 is scattered by diffraction in the first slab waveguide 812 and enters the arrayed waveguides 813 having a plurality of cores 803. Since optical path-length differences are provided between adjacent waveguides of the arrayed waveguides 813, the tilt of a wave front that is propagated through the arrayed waveguide differs depending on the wavelength. The optical signals emitted from the arrayed waveguides 813 are directed into the second slab waveguide 814, where the optical signal is collected for each output channel according to the tilt. The each collected optical signal is then wavelength demultiplexed, and output from the waveguide 815.
As another example of the PLC, a coupler is shown in FIGS. 29 and 30. FIG. 29 is a top view of the coupler; and FIG. 30 is a cross-sectional view taken along the line IXb—IXb in FIG. 29. The coupler has a configuration in which two waveguides (cores 903) are nearby arranged to each other in proximity waveguides area 912 having length L and is widely used as an optical communication device such as light branching, light converging, wavelength filter, or optical switch using thermooptic effect. In the case of light branching, an optical signal entering from input waveguide 911 in FIG. 29 and the adjacent waveguide interfere with each other in the proximity waveguides area 912 having the coupling length L to allow the optical signal to diverge into two. The resultant optical signals are then output from output waveguides 913A and 913B, respectively. The splitting ratio in this case can be changed depending on the length L.
Also in this configuration, as shown in FIG. 30, the upper portions of the two cores 903 formed on the lower cladding 902 on the substrate 901 and the gap between the adjacent cores are covered by the upper cladding 904 in the proximity waveguides area 912.
Currently, there is request to develop an optical waveguide circuit such as the abovementioned AWG or coupler having low insertion loss and having a reduced size. For example, the insertion loss in the PLC device is required to be minimized as much as possible for convenience of system design. At the same time, the size of the device is required to be reduced as much as possible for cost reduction in manufacturing of the device or integration of functions.
In particular, reduction of propagation loss is a common subject in the reduction of the insertion loss in the PLC device. One of the major factors of the propagation loss in the PLC is uneven shape of a boundary surface between the core and cladding, that is, scattering loss due to surface roughness. FIGS. 31, 32 and 33 schematically show the roughness on a boundary surface between core 1003 and upper cladding 1004. FIG. 31 is a top view showing roughness on the core-side surface of the PLC device; FIG. 32 is a cross-sectional view taken along the line Xb—Xb in FIG. 31; and FIG. 33 is a cross-sectional view taken along the line Xc—Xc of FIG. 31. The surface roughness on the core 1003 formed on lower cladding 1002 on substrate 1001 is caused by film surface roughness that has occurred at the time of coating of core layer or pattern roughness due to photolithography and etching at the time of patterning of the core.
In order to reduce the size of the PLC device, it is effective to increase the core-cladding refractive index difference Δ and to decrease the minimum curvature radius of the waveguide. However, in particular, the more the core-cladding refractive index difference Δ is increased, the more the scattering loss tends to be increased. Therefore, when the core-cladding refractive index difference Δ is increased for miniaturization of the device, the core surface must be smoothed for suppressing the scattering loss.
Further, radiation loss arising at a branch point (diverging point) is a major problem particularly in the AWG. The radiation loss at a branch point of the AWG, that is, at a coupling portion between the slab and array accounts for approximately half of the insertion loss in the entire AWG. In order to reduce the radiation loss at a branch portion, it is effective to reduce the distance between the split cores that have branched at a branch point. However, limitation of the accuracy in the photolithography or etching process forces the split cores to be spaced at least about 1 μm apart in general. As shown in FIG. 28, the cladding material is filled in between the cores 803 in general and the boundary between the core 803 and cladding 804 is well-defined. Therefore, most of the signal light that enters the gaps between the sprit cores after propagating through the slab waveguide 812 is introduced into the cladding, which causes the radiation loss. The same can be said for the case where the signal lights enter the slab waveguide 814 from arrayed waveguides 813.
To cope with the problem of the radiation loss arising in the AWG, a publication of patent applications (JP 2000-147283A) has disclosed a configuration in which, as shown in FIGS. 35 and 36 that show cross-sectional views taken along the lines XIb—XIb and XIc—XIc in FIG. 34 respectively, buried layer 1101 having a refractive index higher than the refractive index of claddings 802 and 804 and lower than that of cores 803 is formed between the cores 803 and the thickness of the buried layer 1101 becomes thinner as the distance between the cores becomes wider. With this configuration, the electromagnetic field distribution between the cores 803 at the coupling portion between the slab waveguide 812 and arrayed waveguides 803 is gradually changed to reduce the radiation loss at the branch point. However, in this configuration, the shape greatly depends upon etching condition and consequently, manufacture of a device is difficult, which may result in variation of the shape in a wafer surface or between wafers. Note that, first slab waveguide 812, arrayed waveguides 813, and second slab waveguide 814 are formed between input waveguide 811 and output waveguide 815. Lower cladding 802 is formed on substrate 801.
On the other hand, a problem lies in that the coupling length L of the directional coupler becomes longer especially when the refractive index difference Δ between the core 903 and claddings 902, 904 is increased. That is, although it is effective to increase Δ and decrease the minimum curvature radius of the waveguide for the miniaturization of the device, the increase of Δ strengthens the state where the signal light is confined in the core. Accordingly, the interference to the proximity waveguide is reduced, with the result that it becomes necessary to increase the coupling length L for obtaining a desired splitting ratio. It is possible to reduce the coupling length by narrowing the distance between the proximity waveguides (that is, distance between the cores 903 in the proximity waveguides area 912). However, the distance between the waveguides is restricted by the accuracy in the photolithography or etching process, so that the coupling length needs to be increased.
The present invention has been made to solve the above problems and an object thereof is to reduce loss in the optical waveguide circuit, and to reduce the device size as well as to increase the degree of integration.